The Dunning–Kruger effect is a cognitive bias in which unskilled people make poor decisions and reach erroneous conclusions, but their incompetence denies them the metacognitive ability to recognize their mistakes.[1]
The unskilled therefore suffer from illusory superiority, rating their ability as above average, much higher than it actually is, while the highly skilled underrate their own abilities, suffering from illusory inferiority.

Actual competence may weaken self-confidence, as competent individuals may falsely assume that others have an equivalent understanding. As Kruger and Dunning conclude, "the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others" (p. 1127).[2] The effect is about paradoxical defects in cognitive ability, both in oneself and as one compares oneself to others.

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Although the Dunning–Kruger effect was put forward in 1999, David Dunning and Justin Kruger have quoted Charles Darwin ("Ignorance more frequently begets confidence than does knowledge")[3] and Bertrand Russell ("One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision")[4] as authors who have recognised the phenomenon.

The hypothesized phenomenon was tested in a series of experiments performed by Justin Kruger and David Dunning, both then of Cornell University.[2][5] Kruger and Dunning noted earlier studies suggesting that ignorance of standards of performance is behind a great deal of incompetence. This pattern was seen in studies of skills as diverse as reading comprehension, operating a motor vehicle, and playing chess or tennis.

Kruger and Dunning proposed that, for a given skill, incompetent people will:

tend to overestimate their own level of skill;

fail to recognize genuine skill in others;

fail to recognize the extremity of their inadequacy;

recognize and acknowledge their own previous lack of skill, if they can be trained to substantially improve.

Dunning has since drawn an analogy ("the anosognosia of everyday life")[1][6] to a condition in which a person who suffers a physical disability because of brain injury seems unaware of or denies the existence of the disability, even for dramatic impairments such as blindness or paralysis.

Kruger and Dunning set out to test these hypotheses on Cornell undergraduates in various psychology courses. In a series of studies, they examined the subjects' self-assessment of logical reasoning skills, grammatical skills, and humor. After being shown their test scores, the subjects were again asked to estimate their own rank, whereupon the competent group accurately estimated their rank, while the incompetent group still overestimated their own rank. As Dunning and Kruger noted,

Across four studies, the authors found that participants scoring in the bottom quartile on tests of humor, grammar, and logic grossly overestimated their test performance and ability. Although test scores put them in the 12th percentile, they estimated themselves to be in the 62nd.

Meanwhile, people with true ability tended to underestimate their relative competence. Roughly, participants who found tasks to be relatively easy erroneously assumed, to some extent, that the tasks must also be easy for others.

A follow-up study, reported in the same paper, suggests that grossly incompetent students improved their ability to estimate their rank after minimal tutoring in the skills they had previously lacked—regardless of the negligible improvement in actual skills.

In 2003 Dunning and Joyce Ehrlinger, also of Cornell University, published a study that detailed a shift in people's views of themselves when influenced by external cues. Participants in the study (Cornell University undergraduates) were given tests of their knowledge of geography, some intended to positively affect their self-views, some intended to affect them negatively. They were then asked to rate their performance, and those given the positive tests reported significantly better performance than those given the negative.[7]

Daniel Ames and Lara Kammrath extended this work to sensitivity to others, and the subjects' perception of how sensitive they were.[8]
Other research has suggested that the effect is not so obvious and may be due to noise and bias levels.[9]

Dunning, Kruger, and coauthors' latest paper on this subject comes to qualitatively similar conclusions to their original work, after making some attempt to test alternative explanations. They conclude that the root cause is that, in contrast to high performers, "poor performers do not learn from feedback suggesting a need to improve."[4]

Studies on the Dunning–Kruger effect tend to focus on American test subjects. Template:Cn-span; studies on some East Asian subjects suggest that something like the opposite of the Dunning–Kruger effect operates on self-assessment and motivation to improve:

Regardless of how pervasive the phenomenon is, it is clear from Dunning's and others' work that many Americans, at least sometimes and under some conditions, have a tendency to inflate their worth. It is interesting, therefore, to see the phenomenon's mirror opposite in another culture. In research comparing North American and East Asian self-assessments, Heine of the University of British Columbia finds that East Asians tend to underestimate their abilities, with an aim toward improving the self and getting along with others.[10]

Dunning and Kruger were awarded the 2000 Ig Nobel Prize in Psychology for their report, "Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments".[11]